Simply because Protean is a Hidden Ability. Hidden Abilities aren’t obtained through breeding regularly; it’s like a secret genetic trait. The parent has to have the Hidden Ability too in order for the offspring to have it.
To obtain an HA Froakie, you can go to the chatroom and ask, or look on Reddit or other trading forms.
I’d strongly recommend reading up on Hidden Abilities, since breeding for them varies from each generation.
An important note: the offspring will always be the same Pokemon as the female. I’d personally (in newer games) recommend a male Hidden Ability Froakie and a Female Regular Froakie, but HA being passed on by the gender varies on generation to generation. In Generation 6, only females could pass down Hidden Abilities.
Again, I’d highly recommend reading up on HA breeding for the game you’re playing.
I hope this helps!